QD 

543 

Mas. 


EXCHANGE 


A  STUDY  OF  NICKEL  FERROCYANIDE 
AS  A  MEMBRANE  IN  THE 

MEASUREMENT 
OF  OSMOTIC  PRESSURE 


Dissertation  presented  to  the  Board  of  University  Studies 
of  the  Johns  Hopkins  University  in  conformity  with  the 
requirements  for  the  Degree  of  Doctor  of  Philosophy* 


JOSEPH  LLEWELLYN  MCGHEE 


B.  D.  SMITH  &  BROS.,  PRINTERS,  PULASKI,  VA. 


A  STUDY  OF  NICKEL  FERROCYANIDE 
AS  A  MEMBRANE  IN  THE 

MEASUREMENT 
OF  OSMOTIC  PRESSURE 


Dissertation  presented  to  the  Board  of  University  Studies 
of  the  Johns  Hopkins  University  in  conformity  with  the 
requirements  for  the  Degree  of  Doctor  of  Philosophy* 


JOSEPH  LLEWELLYN  MCGHEE 


MAY,    \  9  \  \ 


ACKNOWLEDGMENT. 


For  the  kind  helpfulness  of  Pres.  Bemsen,  Prof.  Morse, 
Prof.  Jones,  Prof.  Acree  and  Prof.  Whitehead  the  writer  wishes 
to  express  hi«  appreciation,  but  because  so  much  of  inspiration, 
information  through  personal  instruction  and  a  better  apprecia- 
tion of  the  highest  ideals  put  to  practice  in  research  work  came 
to  the  author  through  Prof.  Morse,  it  is  to  him  chiefly  that 
credit  is  due  for  whatever  merit  or  value  may  be  found  in  these 
pages. 

He  is  also  indebted  to  Prof.  Eenouf  for  courtesies  shown. 

For  all  kindness  received  from  his  co-workers  in  the 
laboratory  full  indebtedness  is  hereby  acknowledged. 


251816 


BIOGRAPHY. 


J.  L.  MeGhee,  bora  in  London  County,  Tennessee,  in  1873. 
Beared  mostly  in  Virginia.  High  School  work  at  Pearisburg 
Academy  and  at  Hiwassee.  A.  B.  degree  at  Emory  and  Henry 
College,  Emory,  Virginia,  1903.  Graduate  work  at  the  Uni- 
versity of  Chicago.  Head  of  the  department  of  Chemistry  at 
Emory  and  Henry  College. 


A  STUDY  OF  NICKEL  FERROCYANIDE  AS  A 

MEMBRANE  IN  THE  MEASUREMENT 

OF  OSMOTIC  PRESSURE. 

Dependence  upon  work  previously  done  in  this  laboratory 
for  the  development  of  the  present  investigation  is  fully  recog- 
nized. A  brief  statement  of  this  pre-requisite  work  seems 
necessary  in  order  that  the  connection  of  the  whole  may  be 
properly  set  forth. 

When  the  electrolytic  process  for  the  deposition  of  mem- 
brane was  first  devised,  about  twenty-five  colloidal  substances 
were  tested  to  secure  the  one  best  adapted  to  osmotic  pressure. 
Among  those  showing  most  promise  were  copper  ferrocyanide, 
nickel  ferrocyanide  and  several  others. 

These  tests  were  largely  qualitative  owing  to  the  scarcity 
of  suitable  cells,  manometers,  thermostats  and  other  devices  for 
quantitative  work.  Thus  limited  and  the  task  being  such  a 
stupendous  one,  the  copper  forrocyanide  membrane  was  the  only 
one  used  extensively. 

Although  some  work  had  been  done  to  determine  the 
relative  value  of  other  colloids,  it  had  not  the  advantage  of 
recent  improvements,  until  the  present  year. 

THE  CELL. 

Although  the  preparation  of  suitable  cells  preceded  the 
present  work,  it  is  in  order  to  give  more  than  a  brief  mention  of 
the  troubles  encountered  and  of  the  methods  used  to  overcome 
the  difficulties.  For  obviously  no  membrane  for  measuring 
osmotic  pressure  is  of  any  use  without  a  proper  cell  in  which  to 
place  it.  Likewise  numerous  other  requirements  will  be  noted 
more  or  less  briefly. 

The  ordinary  cell  of  the  potter  does  not  serve  in  measuring 
osmotic  pressure  for  the  following  reasons  or  causes: 

1.  They  do  not  possess  the  strength  necessary  to  withstand 
the  outward  pressure;  most  of  them  crack  under  pressures  below 
20  atmospheres. 

(5) 


2.  They  contain  "air  blisters/'  which  communicate  with 
various  portions  of  the  cell  wall  and  permit  the  formation  of 
irregular,  secondary  membranes,  situated  somewhat  in  parallel 
and  at  varying  distances  from  the  inner  surface.     Any  consider- 
able pressure  brought  upon  a  membrane  deposited  in  such  a  cell 
is  sure  to  break  through  the  colloid. 

3.  The  potter's  cell  also  lacks  uniformity  in  its  pores,  an 
indispensable    property  in    suitable    cells.     This    requirement 
appears  with  more  force  when  it  is  considered  that  the  membrane 
is  deposited  where  the  opposing  ions  meet,  and  that  the  part  of 
the  cell  wall  in  which  they  meet  is  determined  largely  by  the 
size  of  the  pores  in  the  cell.     This  will  be  presented  more  at 
length  under  the  discussion  of  the  membrane. 

To  secure  a  cell  combining  the  necessary  strength  and  uni- 
form porosity  required  several  years  of  persistent  work.  Early 
attempts  to  increase  the  strength  of  the  material  by  adding 
feldspar  were  not  satisfactory  because  uniform  porosity  was  not 
secured  at  the  same  time.  This  was  true  even  when  every  care 
was  used  to  properly  wash,  bolt,  mix,  dry  and  bake. 

Finally  a  suitable  combination  of  clays,  one  deficient  in 
binding  material  and  another  over-rich  in  such  constituents, 
was  tried  with  success;  producing  a  cell  of  uniform  texture  in 
every  respect,  so  far  as  known. 

The  upper  half  of  the  cell  is  glazed,  inside  and  outside. 
For  this  purpose  a  glaze  is  required  whose  coefficient  of  expan- 
sion equals  that  of  the  cell  itself.  A  suitable  glaze  was  finally 
obtained  by  mixing  silica  and  feldspar  with  one  of  the  better 
grades  of  glaze  used  by  the  potter. 

Before  the  cell  is  glazed  it  is  carefully  ground  on  the  lathe 
to  fit  the  brass  collar  and  the  cone  shaped  stopper  of  the 
manometer  attachment.  The  general  shape  of  the  cell  is  that  of 
an  inverted  frustrum  except  at  the  top  where  a  shoulder  is  left 
to  rest  upon  the  collar  that  fastens  together  the  cell  and  the 
manometer.  The  upper  and  lower  surfaces  of  the  shoulder  are 
carefully  turned  on  the  lathe  perpendicular  to  the  longitudinal 
axis  of  the  cell  so  that  the  pressure  from  the  collar  may  be 
evenly  distributed  to  prevent  crushing  when  the  cone  of  the 
manometer  is  screwed  into  place.  Equal  care  is  used  in  cutting 
all  parts  of  the  cell,  which  must  not  be  too  wet  or  too  dry. 

(6) 


The  upper  part  of  the  cell  is  made  larger  to  prevent  the 
outward  pressure  of  the  cone  from  splitting  the  cell,  and  also  to 
avoid  crushing  by  the  cylindrical  collars. 

The  lower  part  of  the  brass  collar  upon  which  the  cell  rests 
during  a  measurement  is  extended  inward  and  covered  with 
lead  in  order  to  afford  a  cushion  for  the  cell  shoulder  under 
pressure,  and  a  cylinder  of  the  proper  size  is  selected  for  each 
cell  so  that  the  cell  shoulder  may  not  be  chipped  off  by  one  that 
is  too  large;  or  that  the  cell  may  not  be  crushed  by  one  that  is 
too  small. 

The  upper  half  of  the  brass  collar  is  quite  heavy.  This  is 
required  because  a  slot  is  left  in  it  to  admit  the  manometer  tube, 
as  the  collar  cannot  be  slipped  over  the  manometer  from  the  top. 
The  middle  portion  of  the  upper  cylinder  is  extended  inward  in 
a  carefully  turned  shoulder  that  rests  on  the  top  of  the  cell.  In 
this  way  pressure  is  applied  without  injury  to  the  cell  or  to  the 
manometer. 

The  brass  cone  of  the  manometer  is  covered  by  rubber 
tubing  of  the  best  quality,  securely  tied  with  several  strands  of 
waxed  shoemaker's  thread  at  the  top  and  bottom,  more  thread 
being  used  at  the  top,  where  the  last  strands  extend  down  to 
the  mouth  of  the  cell,  when  the  cone  is  screwed  into  place. 

(For  a  full  statement  of  the  manufacture  of  cells  employed 
in  osmotic  pressure,  the  reader  is  referred  to  the  American 
Chem.  Journal  of  Feb.,  1911.) 

THE  REMOVAL  OF  AIR  FROM  THE  CELL  WALL. 

All  air  must  be  removed  from  the  pores  of  the  cell  wall 
before  membrane  is  deposited,  in  order  to  secure  uniformity  and 
strength.  This  removal  of  air  is  accomplished  by  the  electric 
current  passing  from  a  platinum  anode  surrounding  the  lower 
unglazed  portion  of  the  cell,  through  an  electrolyte  of  lithium 
sulphate  to  a  platinum  cathode  placed  inside  the  cell.  The 
large  capacity  of  the  lithium  ion  for  water  produces  an  ion  of 
such  size  that  all  air  is  removed  in  about  six  hours, from  a  cell 
of  average  density.  During  this  process  lithium  hydroxide 
Accumulates  at  one  electrode  and  sulphuric  acid  at  the  other. 

(7) 


By  mixing  the  two  solutions  repeatedly  the  lithium  sulphate  is 
again  ready  for  use. 

Before  membrane  is  deposited  the  lithium  sulphate  is 
removed  from  the  cell  wall  electrolytically,  pure  water  being 
frequently  added  to  replace  the  base  and  acid  as  they  accumulate 
about  the  electrodes.  When  the  salt  is  all  removed  the  cell  is 
ready  to  receive  the  membrane. 

THE  DEPOSITION  OF  THE  MEMBRANE. 

A  bath  regulated  to  a  definite  temperature  is  now  required. 
This  is  secured  by  the  use  of  both  gas  and  electricity.  Gas 
supplies  heat  to  raise  the  water  nearly  but  not  quite  to  the 
desired  temperature.  Electric  stoves  with  thermo-regulators 
supply  the  margin  of  difference. 

In  the  bath  a  stirring  apparatus  is  run  by  a  small  motor. 
The  stirrer  is  placed  in  the  upturned  opening  of  an  iron  or  brass 
tube  lying  in  the  bottom  of  the  bath  and  extending  its  entire 
length,  thus  securing  adequate  mixing  of  the  water  and  uniform 
temperature  in  every  part  of  the  bath.  For  higher  temperatures 
the  bath  is  fully  covered  to  prevent  evaporation  and  loss  of  heat. 

Into  glasses  set  in  the  bath  water  are  placed  cylindrical 
nickel  anodes.  A  solution  of  tenth  normal  nickel  sulphate 
nearly  fills  the  glasses.  A  platinum  cathode  and  a  tenth  normal 
solution  of  potassium  ferrocyanide  are  placed  inside  the  cell, 
which  is  provided  with  a  funnel  and  exit  tube  held  in  place  by 
a  two-hole  rubber  stopper.  Through  the  funnel  fresh  potassium 
ferrocyanide  solution,  kept  at  the  bath  temperature,  is  poured 
every  third  minute  during  the  deposition  process.  This  pre- 
vents the  accumulation  of  alkali  inside  the  cell  which  would 
dissolve  the  membrane  if  not  removed. 

The  position  of  the  membrane  in  the  cell  is  a  matter  of 
great  importance.  With  the  very  slow  diffusion  which  would 
exist  in  the  wall  between  a  membrane  in  the  mid- wall  position 
and  the  solution  within  the  cell,  it  would  be  impossible  to 
secure  a  proper  determination  of  osmotic  pressure.  Another 
objection  to  a  mid-wall  membrane  is  that  great  uncertainty 
would  exist  about  the  state  of  such  membrane  in  that  section  of 
the  cell  between  the  outer  and  inner  glaze,  above  the  portion 


which  is  supposed  to  be  the  upper  limit  of  the  membrane  as 
now  deposited.  This  will  appear  more  clearly  from  what 
follows. 

While  a  mid- wall  membrane  is  unsatisfactory,  it  is  equally 
so  with  one  produced  by  surface  precipitation.  In  the  latter 
case  the  membrane  is  not  strong  enough  to  withstand  pressure 
or  to  prevent  flaking  off.  In  order  to  secure  the  membrane  to 
the  cell  wall,  the  porosity  of  the  cell  wall  and  the  voltage  of  the 
electricity  had  to  be  so  adjusted  by  experiment  to  the  rate  at 
which  the  nickel  ion  and  the  ferrocyanogen  ion  penetrated  the 
cell  wall,  that  they  would  meet  just  within  the  wall  surface  on 
the  inner  side,  where  practically  no  hindrance  to  the  diffusion 
of  entering- water,  with  the  solution  in  the  cell,  is  possible. 
Thus  deposited  the  membrane  is  satisfactory  in  every  way  so 
far  as  experience  has  gone.  A  voltage  of  one  hundred,  and 
ten  to  one  hundred  and  fifteen  seems  best  for  this  purpose. 

During  the  deposition  of  the  membrane  in  each  cell  a  record 
of  amperage  is  kept  and  the  risistance  of  the  membrane  in  any 
cell  can  be  determined.  After  receiving  membrane  an  hour  or 
so  the  cell  is  removed,  washed  and  soaked  for  a  day  or  two  in  a 
water  solution  of  thymol  to  remove  all  nickel  sulphate  and 
potassium  ferrocyanide  and  small  amounts  of  any  electrolytes 
that  might  be  present  as  impurities  in  the  cell  wall.  The 
temperature  of  the  cell  is  permitted  to  change  as  little  as  pos- 
sible. Thymol  is  used  as  a  disinfectant  to  prevent  the  growth, 
especially,  of  penicillium,  which  seems  to  feed  upon  the  nitrogen 
of  the  colloidal  compound,  thus  destroying  the  membrane.  The 
cell  is  kept  in  this  thymol  solution  until  it  is  placed  again 
between  the  electrodes  for  more  membrane,  or  till  it  is  ready  to 
be  set  up  for  a  measurement.  Usually  the  first  attempt  to  use  a 
membrane  for  measuring  osmotic  pressure  is  not  successful.  The 
cause  is  supposed  to  lie  in  the  need  that  the  membrane  be  packed 
or  compressed  by  use  until  sufficiently  firm;  repeated  additions 
of  the  colloid  being  made  in  the  meantime.  Thus  the  weakest 
places  are  broken  through  and  are  the  ones  first  filled  or  mended 
when  membrane  is  again  deposited,  as  the  current  passes  through 
the  ruptured  places  most  abundantly. 


(9) 


Repetition  of  the  above  processes  produces  a  suitable  mem- 
brane in  about  two  months,  on  the  average,  if  the  whole  of  the 
large  number  of  necessary  conditions  is  met. 

It  was  found  in  the  present  work  that  commercial  aluminium 
which  was  used  as  racks  for  holding  the  cells  while  in  soak  left  a 
deposit  in  the  thymol  solution.  Tests  showed  this  to  be  in  part 
some  compound  of  aluminium,  probably  the  hydroxide,  and  was 
doubtless  the  result  of  primary  couples  set  up  between  the 
aluminium  and  small  particles  of  other  metals  present  as  im- 
purities. Evidence  of  this  was  found  in  small  pits  that  appeared 
in  the  aluminium.  These  racks  were  discarded.  The  cells  are 
now  placed  in  large  desiccators  which  are  kept  in  the  baths, 
and  provided  with  woolen  hoods  over  the  tops  to  prevent  radia- 
tion of  heat. 

Great  care  is  taken  to  avoid  any  dilution  or  concentration 
of  the  cell  contents.  This  may  occur  either  in  the  failure  to 
have  the  whole  cell  and  its  attachment  at  the  proper  temperature 
when  set  up,  or  it  may  be  caused  by  the  contraction  or  expan- 
sion of  the  mercury  in  the  manometer,  due  to  a  considerable 
difference  between  the  temperature  of  the  bath  in  which  it  is 
placed  during  the  measurement,  and  the  room  temperature  when 
set  up.  It  is  impossible  to  have  the  whole  manometer  at  the 
temperature  of  the  experiment  when  being  set  up,  especially 
while  working  at  very  high  or  very  low  temperatures. 

If  the  cell  and  the  solution  are  too  cool  when  set  up,  expan- 
sion occurs  as  the  proper  temperature  is  reached,  water  passes 
out  of  the  cell,  leaving  the  solution  concentrated.  The  resulting 
pressure  developed  is  higher  than  the  intended  conditions  would 
give. 

On  the  other  hand,  if  the  cell  and  its  contents  are  too  warm 
when  set  up,  contraction  takes  place,  water  is  forced  through 
the  membrane  as  the  cell  is  brought  to  the  cooler  temperature  of 
the  experiment,  resulting  in  dilution  of  the  solution  and  a  lower 
pressure  than  the  true  one  for  the  desired  conditions. 

Every  precaution  is  necessary  to  prevent  leakage  of  the 
solution  from  the  cell  while  under  pressure.  The  various  means 
used  for  this  are  the  following:  The  cell  neck  is  so  ground  as  to 
receive  the  cone  shaped  stopper  which  has  a  definite  taper. 

(10) 


This  cone  is  made  of  heavy  brass.  Through  the  cone  are  passed 
the  capillary  manometer  tube  and  a  small  metallic  tube  which 
connects  with  the  inside  of  the  cell.  After  any  air  bubbles  and 
the  excess  of  solution  pass  out  through  this  tube,  it  is  closed  by  a 
metal  screw  plug  which  fits  upon  an  oil-soaked  washer.  The 
washer  is  kept  from  spreading  by  making  the  contact  surface  of 
the  plug  slightly  concave.  For  temperatures  under  50°  these 
two  tubes  are  fastened  to  the  cone  by  u  Wood's  metal.77  By  use 
of  the  plug  referred  to  and  the  cylindrical  screw  collar  already 
described,  it  is  possible  to  adjust  the  cell  to  any  initial  pressure 
desired.  In  this  way  the  dilution  of  the  solution  which  would 
occur  by  permitting  the  whole  pressure  to  develop  by  osmosis  is 
prevented.  Great  caution  is  used  in  this,  however,  in  order  to 
avoid  too  great  an  initial  pressure. 

After  all  the  precautions  mentioned  above  are  taken,  the 
time  required  for  the  actual  process  of  setting  up  the  cell  is  only 
a  few  minutes.  This  is  essential  for  successful  measurement  of 
osmotic  pressure.  The  manometer  cone  is  forced  into  the  cell, 
screwed  as  tightly  as  desired,  the  nut  inserted  into  the  top  of 
the  small  tube  that  connects  with  the  inside  of  the  cell,  and  the 
cell  is  placed  in  hundredth  normal  solution  of  nickel  sulphate. 
In  the  sugar  solution  within  the  cell  is  placed  an  osmotically 
equivalent  amount  of  potassium  ferrocyanide.  The  liquid  inside 
and  outside  the  cell  is  a  thousandth  normal  solution  of  thymol 
in  water,  to  which  these  other  substances  are  added. 

The  whole  apparatus  is  next  placed  inside  of  an  enclosed 
constant  temperature  bath  described  later  of  such  construction 
that  the  water  below  and  the  air  above  are  both  kept  at  the 
same  temperature. 

THE  MANOMETER. 

Before  the  manometer  is  ready  for  use  an  incredible  amount 
of  the  most  careful  work  must  be  done  upon  it.  Adequate 
description  of  this  part  of  the  work  can  not  be  given  here.  The 
reader  is  referred  to  the  American  Chemical  Journal  of  March, 
1911,  where  the  subject  of  manometers  is  not  only  presented  but 
illustrated  in  a  most  satisfactory  manner  by  Prof.  Morse,  Dr. 
Holland  and  Mr.  Carpenter.  The  only  part  taken  by  the  writer 

(11) 


in  the  manometer  problem  was  vthe  horizontal  calibration  of 
manometers  ©,  ©,  0,  and  @  which  suggested  a  slight  improve- 
ment in  manipulation. 

After  the  manometer  tube  has  the  "  scratch7'  (a  mark  for 
reference)  etched  upon  it,  a  short  column  of  purest  mercury  is 
put  into  the  thoroughly  clean,  dry  tube.  The  latter  is  fixed 
horizontally  upon  the  dividing  engine,  and  over  a  mirror  scale 
of  millimeter  divisions.  Placing  one  end  of  the  short  mercury 
column  even  with  the  "  scratch,"  its  length  is  carefully  deter- 
mined by  use  of  the  calibrating  engine,  provided  with  a  micro- 
scope which  has  a  micrometer  eye-piece.  The  column  of 
mercury  is  shifted  exactly  its  length  along  the  tube,  the  left 
hand  meniscus  resting  where  the  right  hand  meniscus  was  before. 
By  repeating  the  above-  and  weighing  the  mercury  used,  the 
value  of  the  meniscus  correction  between  the  limits  observed 
may  be  calculated. 

To  move  the  mercury  thread  from  one  position  to  the  other 
more  quickly  and  to  adjust  the  meniscus  with  greater  ease  and 
precision  than  had  been  done  before  would  facilitate  a  very 
tedious  work.  It  is  believed  that  the  apparatus  described  below 
not  only  seives  such  a  purpose  but  also  provides  some  security 
against  the  loss  of  the  mercury  thread  when  the  calibration  near 
the  upper  end  of  the  manoneter  is  being  done.  After  several 
days  of  hard  work  one  may  have  it  all  lost  in  a  moment  by  com- 
pressing the  air  too  much  in  moving  the  thread  near  the  end  of 
the  tube. 

A  bulb  is  blown  in  one  end  of  the  manometer  tube  to  be 
used  as  a  mercury  reservoir  later.  To  the  other  end  of  the 
manometer  a  short  piece  of  tube  is  sealed  which  is  also  provided 
with  a  bulb.  Rubber  tubes  are  then  used  to  connect  as  closely 
as  practicable  a  piece  of  glass  tubing  to  each  end  of  the  manom- 
eter tube.  These  connected  tubes  are  bent  at  two  right  angles 
and  return  along  the  front  of  the  calibrating  engine  till  within 
about  ten  centimeters  apart.  Here  the  tubes  are  bent  at  right 
angles  toward  the  operator.  The  projecting  end  of  each  tube  is 
inserted  into  one  end  of  a  rubber  tube  which  connects  them. 
The  rubber  tube  is  longer  than  the  distance  between  the  glass 
tubes  to  allow  for  adj  ustment  of  the  glass  tube  to  manometer 

(12) 


tubes  of  varying  length;  and  also  to  allow  the  operator  full  use 
of  the  rubber  tube  without  in  any  way  displacing  the  manometer. 

At  the  middle  of  the  connecting  rubber  tube  a  small  hole  is 
made.  The  operator  places  his  thumb  and  finger  on  the  tube  over 
this  hole  and  by  properly  compressing  the  air  on  one  side  or  the 
other  the  column  of  mercury  is  forced  along  the  tube  in  either 
direction.  If  too  much  pressure  should  force  the  thread  out  of 
the  capillary,  the  bulb  at  either  end  would  prevent  the  loss  of 
the  mercury  and  save  the  work  already  done. 

Another  advantage  which  is  obvious,  is  that  the  whole 
space  within  the  connected  tubes  is  protected  from  dust  and 
moisture  during  the  several  days  usually  required  for  calibration. 

The  hole  over  which  the  thumb  is  placed  permits  immediate 
equilibrium  of  pressure  as  soon  as  the  thumb  is  removed  and 
prevents  any  tendency  of  the  mercury  to  return  to  aay  former 
position  by  unequal  compression  of  air  on  the  two  ends,  as 
might  occur  if  the  opening  were  not  provided. 

The  whole  apparatus  can  be  made  in  a  few  minutes.  It  is 
simple  and  inexpensive. 

A  perfectly  clean  tube  is  required  for  the  succcessful 
manipulation  of  the  mercury  thread;  otherwise  the  mercury 
u  crawls77  back  toward  the  position  from  which  it  is  moved  for 
a  setting,  resulting  in  serious  errors.  This  "  crawling7'  process 
is  so  slow  that  it  may  occur  unobserved.  Thus  the  worker  may 
exercise  the  greatest  care  and  still  include  serious  errors,  which 
are  not  detected  until  the  duplicate  settings  are  made.  Even 
then  the  same  l  i  crawling 7  7  of  the  mercury  may  produce  similar 
errors  which  would  enable  one  to  get  almost  the  exact  duplicate 
checking  data  and  still  be  far  from  the  true  measurement  of  the 
mercury  thread  lengths  in  the  various  positions  in  the  tubes. 

The  same  trouble  may  arise  from  any  roughening  of  the 
capillary  by  the  cleaning  fluid  used.  This  should  not  remain 
in  the  tube  a  great  while,  in  order  to  avoid  such  trouble.  When 
clean,  the  tubes  are  dried  for  several  hours  by  passing  air 
through  them  over  calcium  chloride  by  use  of  the  suction  pump. 

In  the  calibration  above  referred  to,  the  shorter  the  mercury 
thread  used  the  more  accurately  is  the  volume  of  the  capillary 
determined,  provided  the  weighing  of  the  thread  can  be  very 

(13) 


accurately  done.  In  one  case,  during  the  calibration  above 
referred  to,  an  error  of  three  milligrams  in  the  weight  of  the 
short  thread  of  mercury  resulted  in  a  meniscus  correction  that 
was  one  hundred  per  cent  too  large.  Evidently,  even  with  the 
most  accurate  weighing,  it  is  possible  to  use  a  mercury  column 
that  is  too  short.  In  such  case  any  error  made  is  a  large  per 
cent  of  the  quantity  weighed.  On  the  other  hand,  the  longer 
the  thread  used  the  greater  doubt  exists  as  to  the  size  of  the 
capilJary  between  the  thread  ends.  Threads  about  15  milli- 
meters in  length  were  used  in  this  work.  Three  sets  of  readings 
are  made  for  each  tube,  which  check  within  the  limits  of  experi- 
mental error. 

Another  small  aid  in  this  phase  of  the  calibration  process 
consists  in  a  movable  light-regulator  that  is  attached  to  the 
standard  arm  of  the  engine.  It,  too,  is  quite  simple,  but  saves 
time  in  securing  a  suitable  light  on  the  meniscus  of  the  mercury 
column.  This  is  often  quite  difficult  during  cloudy  weather, 
and  at  certain  positions  the  proper  light  for  a  clear  view  of  the 
meniscus  outline  is  hard  to  obtain  in  good  weather.  This  little 
attachment  consists  of  a  thin  board  or  card-board  that  is  so 
clamped  to  the  arm  of  the  engine  standard  that  it  may  be  freely 
rotated  by  a  light  touch.  This  enables  the  operator  to  shut  off 
or  admit  light  at  any  angle  which  is  found  best  by  trial  for  a 
clear  meniscus.  The  trouble  and  time  thus  saved  far  outweigh 
the  outlay  in  making  it,  because  neither  the  hand  nor  a  card 
can  be  so  conveniently  used.  The  rotating  screen  will  stand  at 
any  angle  desired,  and  can  be  put  on  or  removed  in  an  instant. 

BATHS  FOR  CONSTANT  TEMPERATURE. 

Three  types  of  baths  have  been  used  in  the  osmotic  pressure 
work.  The  first  was  used  for  lower  temperatures  and  has  been 
fully  described  in  previous  reports. 

The  second  has  been  briefly  described  in  these  pages,  and 
perhaps  a  more  full  description  of  the  third  kind  will  embody 
the  essential  features  of  the  second  so  that  it  may  be  better 
understood. 

Since  the  work  from  0°  C.  to  25°  C.  has  been  completed  the 
recently  constructed  baths  are  for  higher  temperatures. 

(14) 


r  °: '  i 

f    UN! 

k  or  J 

X^ft^pyjiHifrX 


A  well  made  box  of  the  size  desired  is  lined  with  copper^ 
tin-coated.  Inside  the  box  is  placed  a  motor-driven  stirring 
apparatus,  which  lifts  the  water  from  the  upturned  opening  of  a 
pipe  extending  from  one  end  of  the  bath  to  the  other.  By 
regulating  the  speed  at  which  the  stirring  is  done,  the  water  in 
all  parts  of  the  bath  may  be  kept  at  a  uniform  temperature. 
For  maintaining  cooler  temperatures  a  system  of  pipes  conveys 
a  constant  stream  of  cold  water  through  the  bath.  This  is 
regulated  by  an  overflow  stand  pipe  which  exerts  constant  pres- 
sure upon  the  water  passing  through  the  bath. 

A  loop  of  the  pipe  through  which  the  stirring  apparatus 
draws  the  water  is  extended  outside  the  end  of  the  bath.  This 
loop  is  heated  by  gas  burners  up  nearly  to  thefcdesired  tempera- 
ture. The  few  degrees  more  required  are  supplied  by  electric 
stoves,  placed  in  cylinders  of  copper  that  extend  into  the  bath, 
or  entirely  from  one  side  to  the  other,  and  are  closed  with  brass 
caps.  To  prevent  sparking  at  the  contact  point  of  the  mercury 
and  platinum  wire  in  the  ther mo- regulator  and  also  on  the  relays 
which  are  used  to  control  the  current,  condensers  are  used. 
These  are  made  of  tinfoil  placed  between  paraffined  paper. 
In  order  to  operate  the  bath  at  different  temperatures  more  or 
less  gas  is  used.  The  small  difference  of  heat  above  referred  to 
is  gotten  from  the  electric  current. 

In  order  to  prevent  evaporation  of  the  bath  water  and  the 
formation  of  mist  on  the  glass  door  through  which  the  readings  of 
the  manometer  are  taken,  it  is  necessary  to  separate  the  upper 
portion  of  the  bath  from  the  lower.  For  this  purpose  a  brass 
plate  covers  the  lower  part  containing  water.  Heavy  weighted 
cans,  open  at  the  top,  extend  through  the  brass  plate  into  the 
water.  The  cans  are  large  enough  and  deep  enough  to  enclose 
the  bottle  containing  the  cell  and  all  the  portions  of  the  manom- 
eter where  the  mercury  reservoirs  are.  The  can  is  then  care- 
fully covered  by  a  thick  heat-insulating  pad,  so  that  only  very 
slight  change  of  temperature  takes  place  when  it  becomes  neces- 
sary to  open  the  bath. 

The  upper  section  of  the  bath  is  so  constructed  that  it  may 
be  removed,  being  clamped  to  the  lower  part  by  sash  locks. 
Rubber  strips  lie  between  the  two  sections  and  insure  close  con- 

(15) 


tact.  A  therino  regulator  controls  electric  lamps  in  the  upper 
portion,  as  below,  and  uniform  temperature  in  the  air  space  is 
secured  by  use  of  an  electric  fan.  For  higher  temperatures  the 
air  space  is  heated  nearly  to  the  desired  temperature  by  a  system 
of  water  pipes  through  which  hot  water  is  circulated  by  a 
stirring  apparatus  similar  to  the  one  described  for  the  main  bath. 
The  pipes  are  situated  at  one  end  of  the  bath  just  behind  the 
electric  fan,  but  are  connected  with  the  stirring  apparatus  and 
the  gas  flames  on  the  outside  of  the  bath.  The  whole  bath  is 
placed  upon  a  support  as  free  from  vibration  as  possible.  Beams 
pass  through  openings  in  the  floor  to  the  wall  below  and  upon 
these  the  bath  is  put.  Three  or  four  such  supports  are  now 
in  use. 

Another  feature  which  avoids  vibration  is  the  method  of 
gearing  the  motor  to  the  water  pump  or  stirrer.  On  the  motor 
shaft  is  placed  a  contact  pulley.  A  belt  transmits  power  to  the 
water  pump.  All  of  this  has  a  separate  support  from  the  bath, 
but  very  little  vibration  is  produced. 

The  cathetometer  used  in  observations  together  with  the 
brass  meter  scale  is  also  supported  on  the  beams  above  referred 
to.  All  manometer  observations  are  referred  to  the  "  scratch" 
or  mark  etched  on  the  glass,  and  the  distance  between  the  upper 
and  lower  limits  of  the  nitrogen  in  the  manometer  is  determined 
by  the  use  of  the  meter  scale  viewed  through  the  telescope  of  the 
cathetometer.  In  these  observations  it  is  necessary  to  bring  the 
telescope  to  a  focus  always  from  the  same  direction  because  of 
the  "  back  lash  "  of  the  adjustment  screw. 

Thus  arranged  the  bath  keeps  constant  temperature  for 
months. 

THE  NICKEL  FERROCYANIDE  CELLS. 

A  more  abundant  supply  of  suitable  cells  recently  made 
practicable  further  investigation  of  other  new,  electrolytically 
precipitated  membranes.  For  purposes  of  record  and  identifi- 
cation the  cells  used  in  this  investigation  are  marked  Nil,  Nia, 
Nis,  and  so  on.  Their  behavior  during  the  removal  of  air  from 
the  cell  walls  and  upon  the  deposition  of  membrane  will  be  noted 
first.  The  cells  used  during  this  work  were  Mi,  Nia,  Ni», 
They  will  be  taken  up  in  the  order  given. 
(16) 


lumber  one  and  number  two  were  started  together  at  the 
beginning  of  the  work. 

The  pores  of  Nil  were  somewhat  less  open  than  those  of  Ni2, 
as  shown  by  the  current  that  passed  in  each  case  when  the  air 
was  being  removed  and  also  when  the  membrane  was  deposited. 
Further  comparison  of  these  two  cells  is  impossible  because  Nil 
was  burst  by  the  manometer  cone  in  being  set  up  before  any 
osmotic  record  for  this  cell  was  established. 

This  was  doubtless  due  to  a  baking  crack  which  was  not 
visible. 

M'iz,  however,  has  the  best  record  of  any  one  of  the  series. 
It  was  prepared  as  indicated  in  the  preliminary  statements. 
From  the  amount  of  current  which  still  passes  through  it  when 
membrane  is  deposited,  it  seems  to  be  the  least  dense  of  the  pix 
cells.  It  was  first  set  up  for  a  trial  measurement  on  Nov.  23, 
1910,  with  a  normal  cane  sugar  solution  ac  25°.  Within  an 
hour  it  had  developed  an  osmotic  pressure  of  approximately 
twenty-five  atmospheres.  From  eight  minutes  past  twelve  to 
one  o'clock  the  mercury  rose  steadily  in  a  closed  manometer 
without  any  oscillation  through  two  hundred  and  twenty-six 
millimeters. 

For  six  days  the  pressure  remained  fairly  constant  but  a 
loss  of  1°.7  occurred  in  the  rotation  of  the  solution  from  the  cell. 
This  was  probably  due  to  the  dilution  caused  by  the  develop- 
ment of  the  entire  pressure  by  osmosis;  resulting  in  the  entrance 
of  a  relatively  large  amount  of  water  and  dilution.  As  a  quan- 
titative test  this  was  not  of  much  value,  but  the  behavior  was 
considered  promising. 

Several  subsequent  attempts  to  secure  a  measurement  failed 
on  account  of  a  leak  at  the  washer  of  the  needle  tube  or  other 
assignable  cause  all  of  which  were  in  no  way  dependent  on  the 
membrane.  For  the  measurements  later  developed  with  this 
cell,  see  experiments  1,  2,  and  5,  on  the  following  pages. 

Nis  met  the  same  fate  that  befell  Kii  and  hence  it  has  no 
record. 

Ni*  of  all  the  cells  showed  the  most  irregular  passage  of  the 
electric  current  in  the  removal  of  air  from  the  cell  and  in  the 
membrane  deposition.  A  few  times  the  resistance  has  reached 

(17) 


abotit  6ne-half  million  ohms,  but  only  after  several  months, 
with  repeated  deposition  of  membrane;  and  this  resistance 
varies.  So  far  no  successful  use  has  been  made  of  Ni4,  though 
recently  it  gave  some  promise  of  more  normal  activity. 

Nie  and  Nis  are  the  only  nickel  ferrocyanide  cells  besides 
Ni2  that  we  have  so  far  used  successfully  to  measure  osmotic 
pressure.  This  does  not  mean,  however,  that  any  one  of  those 
that  are  still  intact  is  unfit  for  such  measurement.  In  this  con- 
nection it  should  be  kept  in  mind  that  if  any  one  of  a  large 
number  of  conditions  is  abnormal,  no  true  measure  of  osmosis  is 
possible.  This  accounts  for  the  relatively  large  number  of 
experiments  which  must  be  performed  in  order  to  produce  and 
maintain  proper  conditions  that  fail  in  no  point. 

Nis  showed  a  dense  texture  throughout.  After  membrane 
had  been  deposited  on  Feb.  2  for  an  hour  and  ten  minutes,  on 
Feb.  3  for  an  hour  and  ten  minutes,  and  on  Feb.  6  for  an  hour 
and  thirty  minutes,  the  cell  showed  a  resistance  recorded  during 
these  three  periods  of  twelve  thousand  ohms.  Comparing  Nis 
with  NJ2  for  similar  conditions,  we  find  the  highest  resistance  in 
Ni2  to  be  fifty-seven  thousand  ohms.  The  least  resistance 
offered  by  Ni2  in  this  time  was  ten  thousand  ohms. 

Nis  was  used  in  experiment  three  herein  recorded.  So  far 
as  known  no  membrane  has  ever  been  developed  in  a  shorter 
time  than  was  required  for  this  one  to  produce  a  measurement. 
This  point  is  more  fully  presented  in  the  general  discussion  of 
the  nickel  ferrocyanide  membrane  later  on. 

Nie  in  its  density  and  resistance  to  the  electric  current  is 
very  similar  to  Nis,  perhaps  as  nearly  alike  as  two  cells  could 
be  found.  In  both  of  these,  the  membrane,  as  shown  by  a 
steadily  decreasing  current,  has  been  deposited  with  regularity 
and  uniformity.  Each  one  offers  about  a  half  million  ohms 
resistance  at  the  end  of  a  brief  deposition  of  fresh  membrane. 

Below  are  presented  several  results  with  the  use  of  the  cells 
just  described: 

EXPERIMENT  I. 

Concentration,  0.1  weight  normal  cane  sugar  solution;  rota- 
tion of  original  solution,  12°. 7;  rotation  at  the  end  of  the 
experiment,  12°. 7;  loss  in  rotation,  Oj  volume  of  nitrogen  in  the 

(18) 


manometer,  454.14;' time  setup,  2  P.  M.,  Jan.  6,  1911;  manom- 
eter, 9;  cell  used  ^12;  resistance  of  membrane,  64,705  ohms; 
initial  pressure  a  little  below  the  theoretical  for  these  conditions; 
pressure  of  mercury,  409;  pressure  of  the  solution,  .01;  capillary 
depression,  .02.  Below  is  table  I.  with  corrections  applied. 

Table  I. 
( Corrected          Pressure 

Time      Temp.    Atmos.  Pres.  (Vol.  of  ETa    Osmotic   Gas    Eatio 

Jan.  7     25°C.  .999  155.69          2.624     2.431   1.080 

u     8         "  .993  155.51          2.635        "       1.084 


Average  1.082 

The  ratio  formerly  established  by  the  Cu2Fe(Cn)e  cells 

for  these  conditions  is     .         .         .         .         .         1.084 

EXPERIMENT   II. 

Concentration,  0.1  weight  normal;  rotation  of  solution 
before  and  after  the  experiment,  12.70;  manometer  9;  cell  used, 
Nia;  set  up  1  p.  M.,  Jan.  13,  1911;  volume  of  N2  in  manometer, 
454.14  calibration  units;  resistance  of  membrane,  70,000  ohms; 
corrections:  liquids  in  manometer,  0.418;  capillary  depression, 
0.02. 

Table  II. 

( Corrected          Pressure 

Time      Temp.  Atmos.  Pres.  ( Vol.  of  N2  Osmotic    Gas  Eatio 

Jan.  15  15°C.  0.991  147.08          2.533     2.349  1.078 

"     16      "  1.010  146.72          2.523         "  1.074 

"     17      "  1.011  146.68          2.523        "  1.074 

u     18      "  1.014  146.61          2.517         "  1.072 


Average        1.006  Average        1.074 

Average  total  pressure,  3.531;  corrected,  2.525. 
When  corrected  by  normal  atmospheric  pressure,  the 

ratio  is 1.078 

The  average  ratio  established  with  the  copper  ferrocy- 

anide  for  the  same  conditions  is  .  1.082 

(19) 


EXPERIMENT  in. 

Concentration,  0.1  weight  normal;  rotation  of  solution  before 
and  after  the  experiment,  12.70;  manometer,  1;  cell  used,  Nis; 
set  up  at  noon  Feb.  18,  1911;  volume  of  N2  in  manometer  is 
445.43;  membrane  resistance,  104,000  ohms;  corrections:  liquids 
in  manometer,  .438;  capillary  depression,  0.02. 


lable  III. 
C  Corrected 


Time      Temp.    Atmos.  Pres. 

(  Vol.  of  ff  • 

Feb.  19  15°C. 

1.001 

144.31 

"     20      " 

0.989 

144.73 

a     21       " 

1.006 

144.47 

"     22      " 

1.001 

144.62 

Pressure 

Osmotic  Gas  Eatio 
2.544  2.349  1.083 
2.547  " 

2.536  " 

2.537  " 


Average        0.999  Average        1.082 

Average  total  pressure  is  3.540;  corrected,  2.541. 
Average  ratio  for  copper  ferrocyanide  is  .  1.082 

EXPERIMENT  IV. 

Concentration,  0.6;  rotation  before  and  after  the  experiment, 
69°.2;  manometer,  ©;  cell  used,  Ni6;  set  up  Mar.  29,  1911,  4  p. 
M.  ;  volume  of  Kz  in  manometer,  445.43;  resistence  of  membrane, 
570,000  ohms;  corrections:  liquids  in  manometer,  0.642;  capil- 
lary depression,  0.02. 

Table  IV. 


(  Corrected 


Pressure 


Time      Temp.    Atinos.  Pres.  ( Vol.  of  N2     Osmotic    Gas    Eatio 


Mar.  30  30° 

"  31  " 

Apr.  1  " 

«   2  '  " 

"   3  " 

u  n 


0.975 
0.988 
0.997 
1.005 
1.009 
1.009 


27.642 

15.802 

14.830 

1.066 

27.578 

15.826 

n 

1.067 

27.508 

15.859 

" 

1.069 

27.556 

15.822 

it 

1.067 

27.561 

15.814 

11 

1.066 

27.574 

15.807 

u 

1.066 

Average  ratio  is     1.067 

Average  for  copper  ferrocyanide  of  this  concentration  at  30° 
yet  to  be  determined. 

(20) 


EXPERIMENT  V. 


Concentration,  0.1  weight  normal;  rotation  of  the  solution 
before  and  after  the  experiment,  12.75;  manometer,  1;  cell  used, 
Ni2j  set  up  4  P.  M.,  Apr.  6,  1911;  volume  of  ^2  in  manometer, 
445.43  ;  resistance  of  membrane,  142,500  ohms  ;  corrections  : 
liquids  in  manometer,  .435  ;  capillary  depression,  0.02. 


Table  V. 
C  Corrected 


Pressure 


Time      Temp.    Atmos.  Pres.  ( Vol.  of  ^2     Osmotic    Gas    Eatio 
Apr.  22     30° 


23 
24 
25 
26 

28 
29 
30 


0.995 

146.99 

2.480 

2.472 

1.004 

1.006 

146.19 

2.486 

u 

1.006 

1.006 

146.44 

2.482 

it 

1.004 

1.006 

146.88 

2.471 

it 

1.000 

1.009 

146.72 

2.472 

u 

1.000 

1.010 

146.52 

2.474 

it 

1.001 

1.000 

147.37 

2.467 

u 

0.998 

0.990 

147.48 

2.473 

it 

0.999 

Average 


1.001 


Average        1.003 
Average  total  pressure  is  3.478. 

Average  total  pressure  corrected  by  the  average  atmospheric 
pressure  is  2.475. 

GENERAL  DISCUSSION. 

One  of  many  requirements  for  a  proper  membrane  to  meas- 
ure osmotic  pressure  is  that  it  shall  not  affect  the  solution  used. 
To  determine  whether  the  nickel  ferrocyanide  causes  any  inver- 
sion of  cane  sugar,  two  tests  were  made. 

The  first  was  by  the  use  of  Fehling  Solution,  which  showed 
no  indication  of  inversion. 

The  second  consisted  in  the  addition  of  some  precipitated 
nickel  ferrocyanide,  which  was  entirely  free  from  other  electro- 
lytes, to  a  sugar  solution  of  known  concentration  and  rotation. 

A  saccharimeter  tube  was  used  in  order  to  take  observations 
from  time  to  time  of  the  rotation.  On  Jan.  16  the  colloid  and  a 
sugar  solution,  whose  rotation  and  concentration  had  just  been 
determined,  were  mixed.  The  rotation  was  retaken  at  once  to 

(21) 


see  the  initial  state  produced.  On  Jan.  17  there  was  no  change 
in  rotation.  On  Jan.  27  no  change  had  occurred,  none  on  Jan. 
30,  and  on  Feb.  13  the  rotation  was  still  the  same.  The  whole 
time  was  28  days.  The  temperature  was  about  20 °C.  The 
sugar  solution  was  taken  from  some  that  had  been  prepared  to 
use  in  a  measurement.  Hence  the  potassium  ferrocyanide  and 
the  thousandth  normal  thymol  solution  were  both  present  just 
as  in  taking  a  measurement.  So  far  as  can  be  seen,  there  is  no 
inversion  of  the  solution  in  the  cell,  caused  by  the  action  of  the 
membrane. 

ACTIVITY. 

When  the  copper  ferrocyanide  membrane  is  used  for  some 
time — two  or  three  years, — its  activity  in  some  cases  falls  off  to 
such  an  extent  that  much  time  is  required  for  the  development 
of  maximum  pressure.  Beca.use  there  are  so  many  possibilities 
of  failure  iu  measuring  osmosis,  the  chances  for  success  are 
greatly  increased  by  shortening  the  time  required  for  reaching 
maximum  pressure.  It  is  quite  possible  that  nickel  ferrocyanide 
as  a  membrane  will  also  come  to  similar  sluggish  action  with 
equal  age.  So  far,  however,  this  tendency  hns  not  appeared  in 
the  oldest  cell,  which  has  been  in  use  seven  months.  By  refer- 
ence to  the  record  of  Ni*2,  some  idea  of  the  activity  of  the  mem- 
brane in  the  beginning  is  obtained.  At  the  present  time  if  a 
cell  is  placed  in  the  bath  for  a  measurement  only  a  few  hours 
elapse  before  a  maximum  pressure  is  exerted. 

As  already  indicated,  a  large  per  cent  of  the  efforts  to 
measure  osmotic  pressure  fail  because  it  is  so  difficult  to  have 
all  necessary  conditions  fulfilled.  One  of  the  chief  difficulties, 
prior  to  the  work  done  in  this  laboratory,  was  the  lack  of  a  good 
membrane  in  a  suitable  cell  and  at  the  proper  place  in  the  cell. 
In  nearly  all  cases  of  failure  to  obtain  satisfactory  results  in  this 
work  with  the  nickel  ferrocyanide  membrane,  the  trouble  has 
been  traced  to  other  causes  than  defects  in  the  membrane  or 
in  the  cell. 

No  special  duration  tests  have  been  made,  but  one  experi- 
ment extended  over  twenty-four  days  with  unimpaired  mem- 
brane at  the  conclusion  of  the  experiment. 

(22) 


MEASURING  Low  CONCENTRATIONS. 

There  is  nob  much  evidence  to  show  that  this  membrane 
responds  promptly  to  barometric  changes,  although  this  property 
may  be  expected  to  accompany  a  membrane  which  shows  such 
activity.  In  measuring  the  pressure  of  dilute  solutions  much 
difficulty  is  found  in  obtaining  concordant  ratios,  due  largely  to 
the  lag  in  the  response  of  the  cell  to  changes  of  atmospheric 
pressure.  The  corrections  made  for  barometric  readings  are  not 
true  expressions  of  the  air  pressure  then  in  effect  through  the 
membrane,  especially  at  times  of  sudden  changes  in  the  weight 
of  the  atmosphere.  Any  error  thus  made  is  so  large  a  per  cent 
of  the  quantity  measured  that  the  relative  effect  on  the  ratio  of 
osmotic  to  gas  pressure  is  large.  In  order  to  equalize  these 
effects  the  average  of  all  barometric  readings  is  used  to  correct 
the  average  of  all  the  total  pressures  exerted.  The  effect  of  this 
is  seen  in  experiment  5. 

This  response  is  doubtless  more  prompt  at  higher  tempera- 
tures. Consequently  less  trouble  would  be  expected  from  this 
cause  at  forty  degrees  than  at  ten  or  fifteen  degrees.  Experience 
bears  this  out. 

The  nickel  ferrocyauide  membrane  was  used  some  during 
the  present  year  to  measure  the  pressure  of  one-tenth  normal 
solutions  due  to  the  greater  lag  of  the  older  copper  ferrocyanide 
membranes  in  responding  to  atmospheric  changes.  And  it  may 
be  that  with  older  nickel  ferrocyanide  cells  the  same  trouble 
would  exist.  This  remains  for  time  to  test.  Since  there  were  also 
new  copper  ferrocyauide  cells  in  use  in  this  work,  the  weight  of 
evidence  is  somewhat  in  favor  of  the  view  that  the  activity  of  the 
nickel  ferrocyanide  membrane  produces  a  more  prompt  response 
to  changes  in  atmospheric  pressure.  This  may  not  prove  to  be 
true  of  all  such  cells,  because  it  is  to  be  noted  that  most  of  this 
work  on  one-tenth  normal  solutions  was  done  with  the  most 
active  of  all  the  nickel  ferrocyanide  cells.  But  here  attention  is 
called  to  experiment  three,  in  which  Nis  was  used.  This  cell, 
as  already  stated,  is  dense  and  any  changes  of  atmospheric  pres- 
sure would  be  slower  in  producing  an  effect  with  Kis  than  with 
the  more  porous  Ni«,  if  cells  of  different  densities  exert  any 
influence  in  the  matter.  But  by  comparing  the  ratios  obtained 

(23) 


in  the  two  experiments,  No.  2  and  No.  3,  those  of  the  more 
dense  and  supposedly  less  active  cell  are  the  more  nearly  con- 
stant, although  other  conditions  were  nearly  the  same. 

Manometer  9  and  manometer  ©  have  been  compared  and 
found  to  agree.  The  variations  of  the  barometer  extended  over 
eleven- thousandth  of  an  atmosphere  in  each  case.  The  duration 
was  the  same  in  both  cases,  both  were  at  15°C.  and  tenth  normal 
concentration.  Again  the  evidence  seems  to  point  to  some 
property  of  the  membrane  for  prompt  action,  but  more  evidence 
is  necessary  before  any  conclusion  can  be  reached. 

In  general  the  resistance  of  the  copper  ferrocyanide  mem- 
brane to  the  current  is  greater  than  that  of  the  nickel  compound. 
Whether  longer  use  and  age  will  produce  a  change  in  this 
respect  is  a  matter  for  future  observation.  Also,  whether  this 
is  an  advantage  or  a  disadvantage  is  not  known. 

By  reference  again  to  the  tables  it  is  seen  that  experiment 
No.  3  shows  an  average  ratio  of  osmotic  to  gas  pressure  of  1.082, 
with  .01  normal  concentration  at  15°C.  But  No.  5  of  the  same 
concentration  with  a  temperature  of  30°  gives  a  ratio  of  unity. 

The  average  ratio  of  all  concentrations  between  0°C.  and 
and  25°C.  is  1.083.  Up  to  25°  stability  seems  to  exist.  At  30° 
and  at  40°  the  ratio  is  unity.  If  the  same  is  found  at  higher 
temperatures  the  Law  of  Boyle  as  well  as  that  of  Gay  Lussac 
will  be  demonstrated  to  apply  to  osmotic  pressure,  as  to  gas 
pressure.  It  has  already  been  demonstrated  by  the  work  be- 
tween 0°  and  25°  that  the  Law  of  Gay  Lussac  holds  between 
those  limits  for  all  concentrations  up  to  the  weight  normal. 
The  final  pressure  of  a  0.1  normal  solution  at  30°  is  the  same  as 
that  which  the  solution  would  be  if  its  volume  were  confined  to 
the  volume  of  the  solvent. 

What  causes  the  change  in  osmotic  pressure  of  dilute 
solutions  between  25°  and  30°  is  not  known.  It  seems  possible 
that  hydration  changes  may  produce  dilation.  Whatever  the 
cause,  the  process  is  one  which  requires  some  time.  At  first  a 
pressure  is  developed  which  nearly  equals  that  at  25°  and  then 
it  gradually  decreases  to  constant  theoretical  gas1  pressure.  In 
several  cases  this  has  been  the  relative  behavior,  and  about  a 
month  is  required  for  a  measurement  of  this  kind.  Experiment 

(24) 


5  was  begun  Apr.  6  and  did  not  develop  constant  pressure  or 
equilibrium  until  Apr.  21.  From  Apr.  21  till  May  1,  there 
was  constant  pressure  as  shown  in  the  table  (5). 

The  element  of  time  required  for  the  deposition  of  mem- 
branes of  sufficient  strength  to  withstand  maximum  pressures  is 
of  some  interest.  Eepeated  subjection  of  the  cell  to  the  current 
through  the  two  solutions,  as  already  described,  is  necessary  in 
order  to  secure  sufficient  stability  or  thickness  of  membrane  to 
withstand  the  higher  osmotic  pressure  exerted  upon  it.  Alter- 
nate soaking  and  depositing  produce  the  desired  results.  The 
exact  effect  of  soaking  the  cells  is  not  definitely  known,  but  it- 
seems  probable  that  practically  all  ionic  or  non-colloidal  material 
is  thus  removed,  resulting  in  a  more  homogeneous  membrane; 
and  also  less  subject  to  attack  by  any  solutions  coming  against 
it.  Summing  up  all  the  time  required  for  the  quickest  deposi- 
tion of  a  copper  ferrocyanide  membrane  and  the  shortest  time 
required  for  a  nickel  ferrocyanide  membrane,  each  giving  a 
measurement,  we  have  the  following  for  comparison:  the  copper 
ferrocyanide  required  28  hours;  the  time  required  by  the  nickel 
ferrocyanide  was  ten  hours  and  forty-six  minutes.  Whether 
any  advantage  besides  the  saving  of  time  is  to  be  derived  from 
this  fact  cannot  be  stated. 

It  will  be  recalled  that  in  depositing  membrane  the  potas- 
sium ferrocyanide  within  the  cell  is  frequently  renewed,  by 
pouring  fresh  solution  through  the  funnel  set  in  the  cell.  The 
funnel  extends  nearly  to  the  bottom  of  the  cell,  and  the  end  of 
the  exit  tyibe  is  just  below  the  rubber  stopper,  so  that  the  fresh 
solution  enters  at  the  bottom,  forcing  the  solution  already  in  the 
cell  upward  and  out  the  exit  tube.  In  this  way  there  is  an 
upward  flow  around  the  electrode.  If  the  amperage  is  noted 
while  the  replacement  of  the  solution  is  taking  place  it  is  found 
that  quite  an  increase  of  current  flows.  In  one  case  where  the 
resistance  before  pouring  in  fresh  potassium  ferrocyanide  was 
110,000  ohms,  it  became  only  55,000  while  the  solution  was 
being  changed.  The  current  in  this  instance  was  doubled. 

The  explanation  suggested  for  this  is  similar  to  that  given 
for  the  rotating  anode  in  electrolytic  work.  But  if  the  solution 
in  the  cell  is  agitated  by  moving  the  funnel  supporting  the 

(25) 


electrode,  no  such  change  is  seen  in  the  current  flow.  In  fact, 
no  difference  in  the  current  is  observed  with  this  treatment. 

When  the  liquid  has  ceased  to  flow  from  the  cell,  after  a 
pouring,  the  current  invariably  decreases  till  it  becomes  the 
same  that  it  was  before  the  change  of  solution,  so  that  the  cause 
can  not  be  in  the  difference  of  the  two  solutions.  It  is  possible 
that  the  force  of  the  flowing  liquid  produces  a  mechanical  dis- 
turbance which  reduces  the  resistance  for  the  time. 

Whether  any  importance  is  to  be  attached  to  this  phenome- 
non is  not  known.  The  facts  are  recorded,  but  the  explanations 
suggested  may  not  be  correct. 

OSMOTIC  PRESSURE  is  NOT  A  FUNCTION  OF  THE  MEMBRANE 

USED. 

Until  the  present  year  only  the  copper  ferrocyanide  mem- 
brane was  used  in  the  quantitative  determination  of  osmotic 
pressure,  and  while  it  has  been  supposed  that  the  nature  of  the 
membrane  was  not  a  factor  in  osmosis,  no  comparison  of  quanti- 
tative results  obtained  by  different  kinds  of  membrane  has  before 
been  possible.  Some  comparisons  of  the  measurements  obtained 
with  both  membranes  appear  in  the  tables.  It  will  be  seen  that 
they  agree  quite  as  well  as  duplicate  measurements  with  the 
same  membrane.  Here  we  have  ample  experimental  evidence 
for  the  conclusion  that  osmotic  pressure  is  not  a  function  of  the 
membrane  used. 


(26) 


M 

251816 


